Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. High voltage discharges between two electrodes excite a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a pulse mode.
Excimer lasers are a particular type of electric gas discharge laser and have been known as such since the mid 1970s. A description of an excimer laser, useful for integrated circuit lithography, is described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled xe2x80x9cCompact Excimer Laser.xe2x80x9d This patent has been assigned to Applicants"" employer, and the patent is hereby incorporated herein by reference. The excimer laser described in Patent ""884 is a high repetition rate pulse laser. In FIG. 1 and FIG. 2, the principal elements of the laser 10 are shown. (FIG. 1 corresponds to FIG. 1 and FIG. 2 corresponds to FIG. 7 in Patent ""884.) The discharges are between two long (about 23 inches) electrodes 18 and 20 spaced apart by about ⅝ inch. Repetition rates of prior art lasers, like the one described, are typically within the range of about 100 to 2000 pulses per second. These high repetition rate lasers are usually provided with a gas circulation system. In the above referred to laser, this is done with a long squirrel-cage type fan 46, having about 23 blades 48. The fan blade structure is slightly longer than the electrodes 18 and 20 and provides sufficient circulation so that at pulse operating rates, the discharge disturbed gas between the electrodes is cleared between pulses. A finned water cooled heat exchanger 58 in FIG. 1 is used to remove heat from the laser gas which is added by the discharge and the fan.
These excimer lasers, when used for integrated circuit lithography, are typically operated on a fabrication line xe2x80x9caround-the-clockxe2x80x9d; therefore, down time can be expensive. For this reason most of the components are organized into modules which can be replaced normally within a few minutes.
Excimer lasers used for lithography must have its output beam reduced in bandwidth to a fraction of a picometer. This xe2x80x9cline-narrowingxe2x80x9d is typically accomplished in a line narrowing module (called a xe2x80x9cline narrowing packagexe2x80x9d or xe2x80x9cLNPxe2x80x9d) which forms the back of the laser""s resonant cavity. This LNP typically is comprised of delicate optical elements including prisms, a mirror and a grating. As repetition rates increase maintaining stable performance by the LNP becomes a serious challenge.
When used as a light source for integrated circuit lithography, the laser beam parameters (i.e., pulse energy, wavelength and bandwidth) typically are controlled to within very tight specifications. This requires pulse-to-pulse feedback control of pulse energy and somewhat slower feedback control of wavelength of the line narrowed output beam. A doubling or more of the pulse rate requires these feedback control systems to perform much faster.
A need exists for gas discharge laser light sources operating at higher average power than prior art devices in order to facilitate increases in production of integrated circuits. For example, there is a need for KrF, ArF and F2 lasers operating at repetition rates in the range of 4,000 Hz to 6,000 Hz and at pulse energies in the range of 5-10 mJ. Higher repetition rates at the above pulse energies create both thermal and radiation challenges inside the resonant cavity of these gas discharge lasers.
What is needed are improvements in the resonant cavity components of these gas discharge lasers to permit high quality performance at these substantially increased average power levels.
The present invention provides an excimer laser with a purged beam path capable of producing a high quality pulsed laser beam at pulse rates in excess of 2,000 Hz at pulse energies of about 5 mJ or greater. The entire purged beam path through the laser system is sealed to minimize contamination of the beam path.
A preferred embodiment comprises a thermally decoupled LNP aperture element to minimize thermal distortions in the LNP. This preferred embodiment is an ArF excimer laser specifically designed as a light source for integrated circuit lithography. An improved wavemeter is provided with a special purge of a compartment exposed to the output laser beam.
Typical prior art excimer lasers of this general type currently in operation are limited to pulse rates of about 1,000 Hz to 2,500 Hz. The present invention provides substantial improvement in laser beam quality and substantial increases in component lifetime as measured by number of laser pulses or total light output. These improvements in beam quality and component lifetime are achieved primarily by assuring an ultra-clean and pure laser beam path and by design improvements to minimize thermal transients while also minimizing the transfer chamber vibrations to the optical components.
This laser is constructed in modular form for quick easy modular replacement for maintenance and repair.